Coronary artery disease, the leading cause of mortality and morbidity in the developed world, is widely treated by implantation of stents in the coronary artery. It is estimated that more than 500,000 Americans and 1,000,000 patients internationally undergo dilation of the coronary arteries by balloon angioplasty and/or stent implantation. However, a major limitation of this revascularization procedure is the high incidence (up to 40-60%; Marx and Marks, 2001; Rensing et al., 2001) of restenosis, the recurrence of constriction of the artery following apparently successful efforts to dilate it, which often occurs within 6 months of the procedure.
Because smooth muscle cell (SMC) proliferation has been strongly implicated in the etiology of in-stent restenosis, many strategies currently being investigated to prevent coronary artery stent restenosis involve coating stents with drugs that inhibit SMC proliferation. The most promising drug evaluated to date for coating stents is rapamycin (sirolimus), an antibiotic that inhibits cell migration and proliferation (Marx and Marks, 2001; Rensing et al., 2001; Sousa et al. 2001). Recent animal and human clinical studies have shown that rapamycin-coated stents are safe and effective in reducing restenosis by more than 90%. Moreover, the luminal diameter of stented coronary arteries actually increased during follow-up in patients with rapamycin-coated stents.
Notwithstanding its early dramatic clinical success in preventing stent restenosis, rapamycin is a first-generation drug and new drugs showing similar or better efficacy with no side effects need to be developed. It has already been demonstrated that prolonged exposure of smooth muscle cells to rapamycin results in the development of resistance to the drug (Luo et al., 1996), suggesting that some patients with implanted rapamycin-coated stents might become resistant to the action of rapamycin and consequently develop restenosis.
Vascular SMC migration is believed to play a central role in the pathogenesis of many vascular diseases, including restenosis after both percutaneous transluminal angioplasty (PTCA) and coronary stenting (Schwartz, 1997). In normal blood vessels, the majority of SMCs reside in the media or middle coat of the vessel, where they are quiescent and possess a “contractile” phenotype, characterized by an abundance of actin- and myosin-containing filaments. In diseased states, however, SMCs migrate from the media to the intima or inner coat of the blood vessel.
Rapamycin, a macrolide lactone, inhibits SMC proliferation both in vitro and in vivo by blocking cell cycle progression at the transition between the first gap (G1) and DNA synthesis (S) phases (Cao et al., 1995; Gallo et al., 1999; Gregory et al., 1993; Marx et al., 1995). The inhibition of cellular proliferation is associated with a marked reduction in cell cycle-dependent kinase activity and in retinoblastoma protein phosphorylation in vitro (Marx et al., 1995) and in vivo (Gallo et al., 1999). Down-regulation of the cyclin-dependent kinase inhibitor (CDKI), p27kip1, by mitogens is blocked by rapamycin (Kato et al., 1994; Nourse et al., 1994).
In p27kip1 (−/−) knockout mice, relative rapamycin resistance was demonstrated, and rapamycin-resistant myogenic cells expressed constitutively low levels of p27kip1 that were not increased with serum withdrawal or addition of rapamycin (Luo et al., 1996).
Rapamycin has been shown to inhibit SMC migration in rats, pigs and humans (Poon et al., 1996). It also has potent inhibitory effects on SMC migration in wild type and p27 (+/−) mice, but not in p27 (−/−) knockout mice, indicating that the CDKI, p27kip1, plays a critical role in rapamycin's anti-migratory properties and in the signaling pathway(s) that regulates SMC migration (Sun et al., 2001).